Safety system having a safety switch

ABSTRACT

A safety system having a safety switch for monitoring a safe protective equipment status of movable protective equipment, having a base housing that comprises at least two part members that can be positioned with respect to one another, wherein a signal receiver is provided in one part member and a signal transmitter is provided in the other part member so that a safe state of the protective equipment is detectable by means of a control and evaluation unit, and having an interlocking device having an electromagnet and a retaining plate that is disposed opposite the electromagnet for locking or unlocking the protective equipment, wherein the control and evaluation unit is configured to unlock or lock the interlocking unit after checking and verifying the signal receiver, wherein a coil of the electromagnet forms a resonant circuit with the capacitor, with the control and evaluation unit being configured to control the resonant circuit cyclically by at least one pulse signal, and with the control and evaluation unit being configured to evaluate the pulse response from the resonant circuit and with at least one contact or with no contact between the electromagnet and the retaining plate being detectable by the control and evaluation unit in dependence on the pulse response.

The present invention relates to a safety system having a safety switchfor monitoring a safe protective equipment status of movable protectiveequipment, having a base housing that comprises at least two partmembers that can be positioned with respect to one another, wherein asignal receiver is provided in one part member and a signal transmitteris provided in the other part member so that a safe state of theprotective equipment is detectable by means of a control and evaluationunit, and having an interlocking device having an electromagnet and aretaining plate that is disposed opposite the electromagnet for lockingor unlocking the protective equipment, and wherein the control andevaluation unit is configured to unlock or lock the interlocking unitafter checking and verifying the signal receiver. Safety switch is asynonymous designation customary on the market for an interlockingdevice or interlocking unit having a lock in accordance with the EN ISO14119 standard.

Machines are used in industry that can be dangerous to humans. Examplesof such machines are presses or robots that perform movements that aredangerous to humans. These zones are secured, for example, by mechanicalfences or light grids. Protective equipment, for example doors, that areprovided with safety switches, is provided in order nevertheless to beable to move into the zone of the hazardous machine. If the door isopened, the safety switch provides for a shutting down of the machine.

Such safety switches furthermore have a lock so that the door is blockedand cannot be opened at all. The lock can only be deactivated to gainentry to the hazardous zone by a stop command at the door or at thecentral human machine interface (HMI) of the machine, with the hazardousmovement being shut down. With machines or processes with trailingmovements, the door has to be locked for so long until the danger in themachine is no longer present.

The doors at machines serves as protective equipment, namely as movableprotective equipment in the sense of the machine guideline.

The monitoring and locking of the door is implemented by safety switcheshaving a lock that are available on the market. A type 2 and a type 4are, for example, known in accordance with the ISO 14119 standard. Theseswitches are based, on the one hand, on a mechanical principle, e.g. thesafety switch i10Lock of SICK corporation, or, for example, on RFIDtechnology on the safety switch having a lock TR10 Lock or MLP1 of theSICK corporation.

An object of the invention comprises providing an improved safety systemhaving an improved safety switch, with it being ensured that the safetyswitch is closed or open.

The object is satisfied in accordance with claim 1 by a safety systemhaving a safety switch for monitoring a safe protective equipment statusof movable protective equipment, having a base housing that comprises atleast two part members that can be positioned with respect to oneanother, with a signal receiver being provided in one part member and asignal transmitter being provided in the other part member so that asafe status of the protective equipment can be detected by means of acontrol and evaluation unit and that comprises an interlocking unithaving an electromagnet and a retaining plate that is disposed oppositethe electromagnet for locking or unlocking the protective equipment,with the control and evaluation unit being configured to unlock or lockthe interlocking unit after checking and verifying the signal receiver,characterized in that a coil of the electromagnet forms a resonantcircuit with a capacitor, with the resonant circuit being acted oncyclically by the control and evaluation unit by at least one pulsesignal and with the pulse response being evaluated by the control andevaluation unit and with at least one contact or with no contact betweenthe electromagnet and the retaining plate being detectable in dependenceon the pulse response.

In accordance with the invention, the coil is acted on by a negativepulse signal in the activated state in which the coil is operated and isprovided with voltage. The coil is, for example, supplied with a voltageof 24 V. The supply voltage of the coil is, for example, cyclicallyswitched off or interrupted for this purpose for a brief time period,for example 5 ms. The switching off or interruption of the voltagesupply takes place cyclically, for example at intervals of 500 ms, sothat the operation of the electromagnet is not impaired by the coil. Thepulse signal can also be called a test pulse or test gap.

The cyclic interruption or switching off of the supply voltage takesplace electronically, for example using an electronic switch. Theelectronic switch can, for example, be a transistor, a transistor stage,or similar. The cyclic interruption is controlled by the control andevaluation unit, for example via an output stage or the transistor.

The pulse response or stop response or pulse function or stop functionof the resonant circuit is evaluated by the control and evaluation unit.The resonant circuit here preferably has a low attenuation so that thestop response displays a transient oscillation behavior in dependence onthe retaining plate close to or in contact with the electromagnet. Thecurve shape of the transient oscillation behavior changes in dependenceon the closeness or on a contact of the retaining plate with theelectromagnet since the coil of the electromagnet is electromagneticallyinfluenced by the metallic retaining plate.

The capacitor in the resonant circuit will, for example, be dischargedfaster during the pulse duration without a retaining plate so that thestop response forms a charge curve of the capacitor having a short timeconstant.

If the retaining plate is a few millimeters remote from theelectromagnet, the capacitor in the resonant circuit is discharged lessfast during the pulse duration, for example, so that the stop responseis a charge curve having a longer time constant.

If the retaining plate is in contact or in electrical contact with theelectromagnet, the capacitor in the resonant circuit is discharged mostslowly during the pulse duration, for example, so that the stop responseis a charge curve having an even longer time constant.

In accordance with the invention, for example, the flank steepness ofthe stop response or a comparison of the stop response can be comparedwith a comparison value or with a threshold value.

The evaluation of the stop response takes place, for example, by asimple transistor circuit or a transistor stage that is evaluated by thecontrol and evaluation unit.

In a further development of the invention, the resonant circuit is aparallel resonant circuit. The coil of the electromagnet is in parallelwith the capacitor here. The parallel resonant circuit has the propertythat the resistance observable at the terminals moves to infinity onresonance.

The parallel resonant circuit has the advantage that the coil or theelectromagnet can be statically operated and the capacitor has so-to-sayno effect in the static operating mode since it is charged and thusforms an almost infinitely high resistance.

In a further development of the invention, an attenuation factor ξ ofthe resonant circuit amounts to at most 0.3 and in particular at most0.2. A resonant circuit is thus used that has a high overshoot behavior.Steep flanks of the pulse response that are simple to evaluate are thusproduced during the short switching off pulses or voltage pulses.

The pulse width depends on the electromagnet, in particular on theinductance of the coil and the capacitance of the capacitor. In afurther development of the invention, the pulse width amounts to amaximum of 6 ms, in particular to a maximum of 5 ms, with the pulseshaving an interval of at least 400 ms, in particular of at least 500 ms.The pulse width thus does not result in a function limitation of theelectromagnet in the active state with an applied voltage. The pulsewidth or pulse length is furthermore sufficiently long to be able tocarry out a simple evaluation. The control and evaluation unit can thusbe configured simply and inexpensively.

In a further development of the invention, an NPN transistor or an NPNtransistor stage is arranged for the evaluation of the pulse response.The NPN transistor is arranged in an emitter circuit, for example. Thevoltage of the electromagnet is here supplied to the NPN transistor asan input signal via a voltage divider. The collector connection isconnected via a resistor to a reference potential of, for example, 3.3or 5 volts. The emitter connector is connected to ground. The collectoris connected to the control and evaluation unit as a signal output

In this respect, a reducing voltage of the pulse response at the baseconnector results in a blocking of the transistor and thus in a positivedigitized voltage signal at the output of the transistor, namely at thecollector-emitter path of the transistor.

The positive response signal of the transistor is the longer, the slowerthe capacitor is charged and the longer the time constant is.

A simple digital output signal at the outlet of the transistor or of thetransistor stage can thus be evaluated by the control and evaluationunit.

The detection of the signal transmitter by the signal receiver and thedetection of the retaining plate by means of the pulse evaluation formtwo diverse signal sources, whereby a diverse and redundant signalevaluation is provided that satisfies a high safety level. The safetyswitch can thus satisfy a safety integrity level of up to SIL3 inaccordance with EC61508/IEC61511.

In a further development of the invention, the signal transmitter andthe signal receiver are configured as an RFID system having an RFIDtransponder and an RFID reader. In this respect, the RFID transponder isarranged at the first part member at which the retaining plate isarranged. The first part member thus forms a passive component withoutits own voltage supply.

The RFID reader is arranged at the second part member at which theelectromagnet is also arranged. The second part member forms an activecomponent having a connector for signal lines and a voltage supply. Itcan be determined by the RFID reader whether the RFID transponder is inthe vicinity of the RFID reader. It can thereby be determined whetherthe first part member is in the vicinity of the second part member.

The invention will also be explained in the following with respect tofurther advantages and features with reference to the enclosed drawingand to embodiments. The Figures of the drawing show in:

FIG. 1 a safety system in accordance with the invention having a safetyswitch;

FIG. 2 an evaluation circuit;

FIG. 3 a supply voltage with voltage interruptions;

FIG. 4 a stop response or pulse function;

FIG. 5 respectively, a stop response and an output signal; and to FIG. 7

FIG. 8 a unit having a safety system.

In the following Figures, identical parts are provided with identicalreference numerals.

FIG. 8 shows a unit 19 having a robot 20 and a hazardous zone 21. Inaccordance with FIG. 8, a safety system 1 is arranged having a safetyswitch 2 for monitoring a safe protective equipment status of movableprotective equipment 3 having a base housing 4 that comprises at leasttwo part members 5 and 6 that can be positioned with respect to oneanother.

The unit 19 can, for example in accordance with FIG. 8, be an industrialunit for manufacturing products. The unit 19 can also be a part of amachine or a unit 19 having a plurality of machines. The unit 19 has ahazardous zone 21 or a safety zone that may not be entered by persons 22when the unit is 19 active since the person 22 may be in danger fromparts of the unit 19. One or more robots 20 whose dangerous active zoneis disposed within the hazardous zone 21 are, for example, arranged inthe unit 19.

The movable protective equipment 3 can be a door or a gate or similarfor securing a hazardous zone.

FIG. 1 shows a safety system 1 having a safety switch 2 for monitoring asafe protective equipment status of movable protective equipment, havinga base housing 4 that comprises at least two part members 5 that can bepositioned with respect to one another, with a signal receiver 7 beingprovided in one second part member 6 and a signal transmitter 8 beingprovided in the other first part member 5 so that a safe status of theprotective equipment can be detected by means of a control andevaluation unit 9, and that comprises an interlocking unit 10 having anelectromagnet 11 and a retaining plate 12 that is disposed opposite theelectromagnet 11 for locking or unlocking the protective equipment, withthe control and evaluation unit 9 being configured to unlock or lock theinterlocking unit 10 after checking and verifying the signal receiver 7,characterized in that a coil 13 of the electromagnet 11 forms a resonantcircuit 15 with a capacitor, with the resonant circuit 15 being acted oncyclically by the control and evaluation unit 9 by at least one pulsesignal and with the pulse response being evaluated by the control andevaluation unit 9 and with at least one contact or with no contactbetween the electromagnet 11 and the retaining plate 12 being detectablein dependence on the pulse response.

In accordance with FIG. 1, the coil 13 is acted on by a negative pulsesignal in the activated state in which the coil 13 is operated and isprovided with voltage. The coil 13 is, for example, supplied with avoltage of 24 V. The supply voltage of the coil 13 is, for example,cyclically switched off or interrupted for this purpose for a brief timeperiod, for example 5 ms. The switching off or interruption of thevoltage supply takes place cyclically, for example at intervals of 500ms, so that the operation of the electromagnet 11 is not impaired by thecoil 13. The pulse signal can also be called a test pulse or test gap.

The cyclic interruption or switching off of the supply voltage VDD, thatis shown in FIG. 3, takes place electronically, for example using anelectronic switch. The electronic switch can, for example, be atransistor, a transistor stage, or similar. The cyclic interruption iscontrolled by the control and evaluation unit 9, in accordance with FIG.1, for example, via an output stage or the transistor.

The resonant circuit 15 from FIG. 1 is shown in more detail in FIG. 2.FIG. 2 shows the supply voltage VDD, the stop response VS, and theoutput signal VA.

The pulse response or stop response VS or pulse function or stopfunction of the resonant circuit, that is shown in FIG. 4, is evaluatedby the control and evaluation unit 9. The resonant circuit 15 herepreferably has a low attenuation so that the stop response VS displays atransient oscillation behavior that can be evaluated in dependence onthe retaining plate 12 close to or in contact with the electromagnet 11.The curve shape of the transient oscillation behavior changes independence on the closeness or on a contact of the retaining plate 12with the electromagnet 11 since the coil 13 of the electromagnet 11 iselectromagnetically influenced by the metallic retaining plate 12.

FIG. 5 shows the stop response VS and a digitized output signal VA forthe event that no retaining plate is in the vicinity of theelectromagnet. The digitized output signal VA is formed on the basis ofa threshold comparison with the stop response VS and is evaluated by thecontrol and evaluation unit 9. As soon as the signal of the stopresponse VS has fallen below a specific level, the output signal VA hasa positive level. The output signal VA thus has a first longer pulse anda second shorter pulse in accordance with FIG. 5.

The capacitor 14 in the oscillating circuit 15 will, for example, bedischarged faster in accordance with FIG. 2 during the pulse durationwithout a retaining plate 12 so that the stop response VS forms a chargecurve of the capacitor 14 having a short time constant in accordancewith FIG. 5.

The control and evaluation unit 9, that is, for example, configured as amicrocontroller, triggers the 5 ms long pulse at intervals of 500 ms andmonitors the output signal VA or the output voltage. The control andevaluation unit 9 or the microcontroller checks the output signal VA,for example by polling, by software, or by an interrupt routine bysoftware or hardware.

FIG. 6 shows the stop response VS and a digitized output signal VA forthe event that no retaining plate 12 is in the vicinity of theelectromagnet 11. The digitized output signal VA is formed on the basisof a threshold comparison with the stop response VS and is evaluated bythe control and evaluation unit 9. As soon as the signal of the stopresponse VS has fallen below a specific level, the output signal VA hasa positive level. The first longer pulse of the output signal VA isextended in accordance with FIG. 6 with respect to the output signal VAin accordance with FIG. 5.

If the retaining plate 12 is a few millimeters remote from theelectromagnet 11, the capacitor 14 in the oscillating circuit 15 isdischarged less fast in accordance with FIG. 2, while the pulseduration, for example, is discharged less fast so that the stop responseVS is a charge curve having a longer time constant, in accordance withFIG. 6.

If the retaining plate 12 is in contact or in electrical contact withthe electromagnet 11, the capacitor 14 in the resonant circuit 15 isdischarged most slowly during the pulse duration, for example, so thatthe stop response VS is a charge curve having an even longer timeconstant, in accordance with FIG. 7.

FIG. 7 shows the stop response VS and a digitized output signal VA forthe event that no retaining plate 12 is in contact with theelectromagnet 11. The digitized output signal VA is likewise formed onthe basis of a threshold comparison with the stop response VS and isevaluated by the control and evaluation unit 9 in accordance withFIG. 1. As soon as the signal of the stop response VS has fallen below aspecific level, the output signal VA has a positive level. The firstlonger pulse of the output signal VA is extended in accordance with FIG.7 with respect to FIG. 6 and is now only a single longer signal.

If the control and evaluation unit 9 detects the contact between theelectromagnet 11 and the retaining plate 12, a pulse signal 18 is nolonger generated by the control and evaluation unit 9, for example, anda continuous supply voltage VDD of, for example, 24 volts is applied tothe electromagnet 11 without interruptions.

If the safety switch 2 is opened in accordance with FIG. 1, which isdetectable, for example, by the signal receiver 7, the control andevaluation unit 9 can again cyclically apply the pulse signal 18 to theelectromagnet 11 to again detect a contact with the retaining plate 12.

In accordance with FIGS. 5 to 7, for example, the flank steepness of thestop response VS or a comparison of the stop response VS can be comparedwith a comparison value or with a threshold value.

The evaluation of the stop response VS takes place, for example, by asimple transistor circuit accordance with FIG. 2 or a transistor stagethat is evaluated by the control and evaluation unit 9.

In accordance with FIG. 2, the resonant circuit 15 is a parallelresonant circuit. The coil 13 of the electromagnet 11 is in parallelwith the capacitor 14 here.

The parallel resonant circuit has the advantage that the coil 13 or theelectromagnet 11 can be statically operated and the capacitor 14 hasso-to-say no effect in the static operating mode since it is charged andthus forms an almost infinitely high resistance.

In accordance with FIG. 2, an attenuation factor ξ of the resonantcircuit 15 amounts, for example, to at most 0.3 and in particular to atmost 0.2. A resonant circuit 15 having a high quality is thus formed.Steep flanks of the pulse response that are simple to evaluate are thusproduced during the short switching off pulses or voltage pulses inaccordance with FIGS. 5 to 7.

In accordance with FIG. 3, the pulse width amounts to a maximum of 6 ms,in particular to a maximum of 5 ms, with the pulses having an intervalof at least 400 ms, in particular of at least 500 ms. The pulse widththus does not result in a function limitation of the electromagnet 11 inthe active state with an applied voltage. The pulse width or pulselength is furthermore sufficient to be able to carry out a simpleevaluation.

In accordance with FIG. 2, an NPN transistor 16 is, for example,arranged for the evaluation of the pulse response. The NPN transistor 16is arranged in an emitter circuit. The voltage of the electromagnet 11is here supplied to the NPN transistor as an input signal via a voltagedivider. The collector connection is connected via a resistor to areference potential of, for example, 3.3 V. The emitter connector isconnected to ground. The collector is, in accordance with FIG. 1,connected to the control and evaluation unit 9 as a signal output

In this respect, a reducing voltage of the pulse response VS at the baseconnector results in a blocking of the transistor 16 and thus in apositive digitized voltage signal or output signal VA at the output ofthe transistor 16, namely at the collector-emitter path of thetransistor 16.

The positive output signal VA of the transistor 16 is the longer, theslower the capacitor 14 is charged and the longer the time constant isin accordance with FIGS. 5 to 7.

A simple digital output VA signal at the outlet of the transistor 16 orof the transistor stage can thus be evaluated by the control andevaluation unit 9.

In accordance with FIG. 1, the signal transmitter 8 and the signalreceiver 7 are configured, for example as an RFID system having an RFIDtransponder and an RFID reader. In this respect, the RFID transponder isarranged at the first part member 5 at which the retaining plate isarranged. The first part member 5 thus forms a passive component withoutits own voltage supply.

The RFID reader is arranged at the second part member 6 at which theelectromagnet 11 is also arranged. The second part member 6 forms anactive component having a connector for signal lines and a voltagesupply. It can be determined by the RFID reader whether the RFIDtransponder is in the vicinity of the RFID reader. It can thereby bedetermined whether the first part member 5 is in the vicinity of thesecond part member 6.

REFERENCE NUMERALS

1 safety system

2 safety switch

3 protective equipment

4 base housing

5 first part member

6 second part member

7 signal receiver

8 signal transmitter

9 control and evaluation unit

10 interlocking unit

11 electromagnet

12 holding plate

13 coil

14 capacitor

15 resonant circuit

16 transistor

17 RFID system

18 pulse signal

19 unit

20 robot

21 hazardous zone

22 person

VDD supply voltage

VS stop response

VA output signal

The invention claimed is:
 1. A safety system having a safety switch formonitoring a safe protective equipment status of movable protectiveequipment, the safety system comprising: a base housing that comprisesat least first and second part members that can be positioned withrespect to one another, wherein a signal receiver is provided in thefirst part member and a signal transmitter is provided in the secondpart member so that a safe state of the protective equipment isdetectable by means of a control and evaluation unit, and aninterlocking device having an electromagnet and a retaining plate thatis disposed opposite the electromagnet for locking or unlocking theprotective equipment, wherein the control and evaluation unit isconfigured to unlock or lock the interlocking unit after checking andverifying the signal receiver, wherein a coil of the electromagnet formsa resonant circuit with a capacitor, with the control and evaluationunit being configured to control the resonant circuit cyclically by atleast one pulse signal, and with the control and evaluation unit beingconfigured to evaluate the pulse response from the resonant circuit andwith at least one contact or with no contact between the electromagnetand the retaining plate being detectable by the control and evaluationunit in dependence on the pulse response.
 2. The safety system inaccordance with claim 1, wherein the resonant circuit is a parallelresonant circuit.
 3. The safety system in accordance with claim 1,wherein a damping factor ξ amounts to at most 0.3.
 4. The safety systemin accordance with claim 3, wherein the damping factor ξ amounts to atmost 0.2.
 5. The safety system in accordance with claim 1, wherein apulse width of the pulse signal amounts to a maximum of 6 ms.
 6. Thesafety system in accordance with claim 5, wherein the pulse width of thepulse signal amounts to a maximum of 5 ms.
 7. The safety system inaccordance with claim 1, wherein an NPN transistor or an NPN transistorstage is arranged for the evaluation of the pulse response.
 8. Thesafety system in accordance with claim 1, wherein the signal transmitterand the signal receiver are configured as an RFID system having an RFIDtransponder and an RFID reader.